Development of electrostatic patterns with aqueous conductive developing liquid



Dec. 30. 1969 5 ET AL 3,486,922

DEVELOPMENT OF ELECTROSTATIC PATTERNS WITH AQUEOUS CONDUOTIVE DEVELOPINGLIQUID Original Filed Nov. 18, 1965 2 Sheets-Sheet 1 FIG. 70 FIG. 7b

9 eay" V/cm F/G.2G

AQUEOUS DROPLET INSULATING LAYER g BEFORE w CHARGING n M comoucrwaSUPPORT F/OZD AQUEOUS DROPLET INSULATING LAYER BEFORE CHARGING ffiCONDUCTIVE 5 SUPPORT 2c INVENTORS PAUL MAR/A CASS/ERS ROBERT JOSEPH NOEJOZEF LEONARD VAN ENGELAND WATSON, COLE GRINDLE & WA TSON ATTORNEYS Dec.30. 1969 P, s ET AL 3,486,922

DEVELOPMENT OF ELECTROSTATIC PATTERNS WITH AQUEOUS CONDUCTIVE DEVELOPINGLIQUID Original Filed Nov. 18, 1965 2 Sheets-Sheet 2 AC OR 0c v0 LTA G ESOURCE ELECTROSTATIC DEVELOPMENT WITH HYDROPHOBIC RESIDUE CHARGE IMAGEAQUEOUS DEVELOPER FROM DEVELOPMENT CHARGE uoum' CARR'iER") 1 5 1 1 i Z 1I 2 i F/G.4Cl F/G.4b F/G.4C

CHARGE PATTERN RE'DEVELOPMENT WITH AFTER RE'CHAR'GING AQUEOUS DYESTUFFSOLUTION r-" -"s T I i I F/G.4d F/G.4

INVENTORS PAUL MARIA cAss/E s ROBERT JOSEPH NOE JOZEF LEONARD VANENGELAND WATSON, COLE, GR/NDLE 8 WA TSON ATTORNEYS United States PatentInt. Cl. G03g 13/02 US. Cl. 117-37 12 Claims ABSTRACT OF THE DISCLOSUREElectrostatic charge patterns on normally insulating electrostaticcharge carriers having a hydrophoblic surface are developed by means ofsurface wetting with a liquid. An aqueous conductive developing liquidis brought into substantially uniform contact with the hydrophobicsurface of the carrier in the presence of a superimposed direct currentelectric field extending generally at right angles to the surface. Thehydrophobic surface is normally unwettable by the aqueous developingliquid, but under the influence of attractive electrostatic field forcesprovided by the charge pattern and the superimposed field, the liquid isselectively attracted into wetting relation therewith. The period ofdevelopment contact is less than the discharge time of the chargepattern upon contact of the carrier with the liquid. The contact mayoccur over the entire surface or progressively across the surface, andwhere the charge carrier has rectifying characteristics, the field canbe created by alternating current.

This application is a continuation of US. patent application Ser. No.508,486 filed Nov. 18, 1965, now abandoned, which was acontinuation-in-part of Us. patent application Ser. No. 150,820 filedNov. 7, 1961, now abandoned.

Many image and signal recording processes are known which involve thecreation of an electrostatic record from or by means of which a visiblerecord can be subsequently be formed. We refer by way of example ofelectrostatic printing processes (see e.g. United Kingdom patentspecification 618,512), electrostatic recording processes, xerography(see e.g., United Kingdom patent specification 672,767), the Electrofaxprocess as described by C. J. Young and H. C. Greig in RCA Rev. (1954)469, electrothermography as described by P. M. Cassiers, J. Soc. Phot.Sci. Eng. 4 (1960) 199.

Many methods of developing latent electrostatic records have beenproposed. We mention in particular, methods in which the electrostaticrecord is developed to a powder image by the electrostatic attraction ofpowder dusted over the electrostatic record or by the deposition ofpowder by electrophoresis from dispersion in a liquid with a highelectric resistivity. For the latter method reference is made e.g. tothe United States Patent 2,913,353.

Although powder development gives good results, it is open to variousobjections. These arise in part from the inconvenience of working upsuitable dry powders and the ice necessity for a special fixing stepafter deposit of the powder, e.g., heating or application of solvents,so as to make the image permanent. Moreover, in order to obtain uniformand equivalent image reproduction with powder development, a ratherextensive and expensive installation has to be used.

When developing with dispersions in liquids with high electricalresistivity, the choice of suitable liquid media is limited.Electrically insulating liquids e.g. chlorinated hydrocarbons or highpurity water have to be used.

The present invention, which will be defined hereinafter, is based inpart, on the discovery that if an electrostatic charge carrier having ahydrophobic surface and bearing an electrostatic charge patterndetectable at such surface is contacted non-differentially (as willhereafter be defined), with an appropriately chosen liquid medium, theelectrostatic charges influence the interfacial tension between thehydrophobic surface and the liquid medium so that the liquid wets thesurface according to the charge pattern. In consequence a record interms of surface wetting and indicative of an electrostatic record inthe form of image-wise or signal-wise deposited or induced electrostaticcharges borne by a support can be formed by simply flooding or otherwisesupplying the whole support with liquid.

The expression hydrophobic where used in this specification in relationto a charge carrier surface means that water forms on the said surface(in the absence of attractive electrostatic charges) a contact angle ofat least (For a definition of contact angle see J. Alexander, ColloidChemistry, vol I, Principles and Applications, 4th ed., D. van NostrandCompany, Inc., New York, pp. 79-80.)

For recording the electrostatic charge pattern a liquid medium is usedwhich does not normally wet the hydrophobic surface of the chargecarrier, or not to any appreciable extent, but which will wet suchsurface under the influence of the encountered electrostatic attractivecharges so as to record the electrostatic charge pattern in terms ofsurface wetting. In the interest of forming good quality records, it ispreferably for the electrostatic charges to be capable of reducing thecontact angle between the liquid and the hydrophobic surface to such anextent that complete or spreading wetting occurs (contact angle of 0 orapproximately so). Water together with one or more other ingredients(e.g., an organic polar liquid), can be used provided that thecomposition as a whole is sufiiciently electrically polarizable, sopreferably electrically conductive. When using a mixture of liquids, theminimum proportion of water required will depend upon the properties ofthe other ingredient or ingredients. If an organic liquid with highdielectric constant such as formamide is incorporated, this may bepresent in sub stantial amount, e.g., 60% by weight, but in general itis preferred in all cases to use a liquid comprising at least 60% byweight of water and with a specific conductivity of at least 10 mho/cm.As will hereafter be illustrated, the liquid used in the process can bea solution. It is also possible to use a liquid which contains dispersedmaterial, e.g., to use an aqueous liquid in which a finely dividedmaterial is dispersed, forming a stable suspension or emulsion. However,in view of the known electrophoretic development technique alreadyreferred to herein it is to be understood that we lay no claim in thisspecification to the use of insulating liquid being a suspension oremulsion of such constitution that the electrostatic charges causedispersed solid or liquid to move through the continuous phase and thusto increase in concentration at the areas where this continuous phaseliquid Wets the charge carrier. It has already been mentioned that inthe electrophoretic development the choice of suitable liquid media islimited. In electrophoretic development the objective is always to bringabout displacement of the dispersed material (which constitutes thedeveloper) through the continuous liquid phase under the influence ofelectrostatic charges constituting a latent image. Taking into account arather long sedimentation time of the dispersed particles it isessential in electrophoretic development to use as carrier for thedispersed developer, a liquid which is electrically insulating thuspreventing the carrying off of the electric charges of the electrostaticimage. In British patent specification 755,486 for example, whichdescribes electrophoretic development, it is stated that apart from electrically insulating organic liquids, Water can be used on theunderstanding that it is of high purity.

It has now been found that record-wise wetting of an electrostaticallycharge containing hydrophobic dielectric medium, preferably aphotoconductive layer containing an electrostatic charge image, with aconductive aqueous liquid can be obtained on the electrostaticallyliquid attracting areas of that medium by non-differentially contactingsaid medium with said liquid for a period not succeeding the dischargetime of the charge which will be dissipated through that liquid,preferably within a time interval not greater than the time interval inseconds at which the original charge is diminished to the l/e value, ebeing the base number of Napierian logarithms.

A second discovery on which the present invention is based is that thequality of a record in terms of surface wetting formed as abovedescribed and the relationship between the record and the electrostaticcharge pattern, i.e., whether these are in positive to positive orpositive to negative relationship, is influenced if a biassingelectrical potential of suitable magnitude is applied between electrodesextending over the area of the liquid/surface contact zone, on oppositesides thereof, so as to superimpose an electric field which traversesthe surface normally thereto.

The superimposed field may be a direct current electric field or, if thecharge carrier has rectifying properties, an alternating current field.If a direct current field of appropriate strength with a directionopposite to that of the field due to the electrostatic charge pattern issuperimposed, the effect is to improve the quality of the record, and inparticular of giving records with better contrast, whereas bysuperimposing a direct current electric field of appropriate voltage inthe same direction as the field due to the electrostatic charge patternthe effect is that the liquid is inhibited from or restrained in wettingthe electrostatically charged or image areas of the surface and iscaused to wet such surface in the other areas where there is no ornegligible electrostatic charge thereby forming a reversed or counterimage. In the event that the charge carrier has rectifying propertiesand an alternating current field of appropriate strength and frequencyis superimposed, an effective bias is created which assists or retardswetting in a given area of the charge carrier depending on thecomposition of the carrier. When using a carrier comprisingphotoconductive zinc oxide, a counter image is obtained while withmaterials causing rectification in the reversed sense a non-reversedimage of improved quality (sharper contrast) is surprisingly obtained.It is preferable to use an alternating current field with a frequencysubstantially above the normal 50 cycles, say a frequency of 100 cyclesor more, and we have so far obtained the best results with frequenciesabove 500 cycles, e.g., 1000 cycles, all frequencies mentioned hereinbeing in cycles per second.

Accordingly the present invention includes any process in which ahydropholic surface of an electrostatic charge carrier bearing anelectrostatic charge pattern which may be a persistent internalpolarization image (see Advances in Xerography 8l962) PhotographicScience and Engineering, vol. 7, 1963 pp. 9 and 10) detectable at suchsurface is wetted with liquid according to such charge pattern therebyto record such pattern in terms of surface wetting, by contacting saidsurface nondifferentially (as herein defined), and either or not withina superimposed direct or alternating current electric field extendingnormally such surface, with an aqueou conductive liquid with which suchsurface is normally substantially unwettable but by which such surfaceiS wettable under the influence of attractive electrostatic charges.

The expression electrostatic charge pattern is used broadly to includeany distribution of charges within a support area such that differentparts of this area are charged to different extents or such that someparts only are charged. Thus the expression includes electrostaticimages of reading matter, diagrams, pictures etc., and chargesconstituting the electrostatic record of wireless or other signals.

By the expression non-differential as used herein in relation to thecontact of the charge carrier surface by the liquid we mean that saidsurface is contacted by the liquid alike in the charged and unchargedareas, or, if the electrostatic charge pattern results from overall butdifferential charging of the carrier, then alike in the differentiallycharged areas. This non-differential contact may be brought about bysupplying liquid onto the whole surface of the charge carrier or bycontacting the surface with the liquid at a multiplicity of closelyspaced points or lines uniformly distributed over the carrier surface.The aqueous liquid medium for the development may so be supplied to thecharge carrier by a liquid supply means wherein such medium is held in acapillary recess or passage or a plurality of capillary recesses orpassages.

From what has already been said as to the different relationships inwhich the electrostatic charge pattern and the record in terms ofsurface wetting may stand to each other, it will be understood that whenin this specification reference is made to the wetting of a surfaceaccording to an electrostatic charge pattern, it is meant that thedeposited liquid pattern visibly or detachably records the subject ofthe electrostatic record. The liquid and electrostatic records need notcorrespond in the sense that two positive records of a text correspond;on the contrary one may be positive and the other negative.

The expression counter image is used herein to devote an image which isreversed in this sense with respect to electrostatic image.

By way of further explanation, it is not necessary, as will be clearfrom the examples of the invention to be given hereafter, for the wholecharge carrier surface to be non-differentially contacted with theliquid at one and the same time. For example the liquid can be appliedto the surface by means of a roller which moves progressively thereover.In such cases, the superimposed electric field need not extend acrossthe whole of the charge carrier surface during the whole of the periodin which liquid is being applied to such surface but can at any givenmoment merely extend substantially uniformly across the area of thecharge carrier surface in which contact with liquid is at that momenttaking place and the said field may move progressively along the saidsurface with the liquid applicator. The claims hereof are to beconstrued accordingly.

The new technique of depositing liquid in dependence on an electrostaticcharge pattern in a superimposed electric field may be applied to give aone-step development process in which, by the application of developingliquid, a visible image which does not require a special after treatmentfor fixing purposes is created. Thus, for example, the aqueous mediumused in a process acc0rding to the invention as above defined may be anink or dye or a medium which contains an ingredient which reacts, e.g.,with a component present in the charge carrier or with the atmosphere,to form a dye. The invention is not however limited to the directproduction of dye or other visible or final records from anelectrostatic record. The new techniques opens the way to a variety ofalternative ways of using such a record. As one example, afterdepositing an uncoloured hydrophilic composition on the hydrophobiccharge carrier surface under the influence and according to the patternof electrostatic charges thereon the resulting liquid deposit can beconverted into a readily visible image by passing over the surface aroller charged with an aqueous dyestuif solution. Various other ways ofindirectly forming a visible record starting with an electrostaticrecord will be described hereinafter. It is also to be understood thataque ous medium deposited under the influence of the electrostaticcharge pattern can be transferred to another support for the purpose ofproducing a visible record on that support in the same or by asubsequent step.

The particular method by which the electrostatic charge pattern isformed is not in any way crucial. By way of example the techniques forproducing electrostaitc records as summarised in the introductoryparagraphs of this specification can be used, or any method, e.g.,electrical polarization of an insulating material. The invention has sofar however been primarily developed in connection wiihelectrophotographic processes in which the electrostatic record iscreated in a photoconductive layer as a result of the application of anelectric charge or field on record-wise exposure of the layer to lightor other radiation which raises the electrical conductivity of thelayer.

In a process according to the invention non-differential supply ofliquid to the charge carrier may be effected in a variety of ways. Amethod will later be illustrated, which involves the progressive contactof the whole electrostatic record area with a continuous liquid face.

As a carrier for an electrostatic latent record to be used in a processaccording to the invention insulating layers or sheet containingphotoconductive or thermoconductive substances may be used. Thesematerials preferably comprise a back-layer or a support, which possesseshigher conductivity than the insulating layer.

According to a preferred feature in a process according to the presentinvention, a photoconductive layer with hydrophobic properties is usedas carrier for the electrostatic charge pattern. Preferably thephotoconductive layer comprises a photoconductor dispersed in aninsulating binding agent. The usual photoconductive layers whichcomprise organic or mineral photoconductive substances incorporated intoinsulating polymeric binding agent, and the photoconductive layers whichcomprise organic photoconductive polymers, possess a hydrophobiccharacter. Examples of photoconductive layers comprising photoconductivepolymers are described e.g. in United Kingdom patent specifications964,877 and 964,875. Examples of photoconductive binding agents moreespecially for photoconductive zinc oxide are described in UnitedKingdom Patent 964,885.

As an illustration, a nonlimiting list of hydrophobic polymeric bindingagents for photoconductive materials is niven here:

Polyvinyl acetate Copolymer of vinyl acetate and an ester of vinylalcohol and a higher aliphatic carboxylic acid such as lauric acid,stearic acid, palmitic acid, e.g. co[vinyl acetate/ vinyl stearate](85/15) Polyalkylmethacrylate, e.g. Plexigum P26 (trademark for anacrylic resin marketed by Rohm Haas G.m.b.I-l., Darmstadt, Germany)Kunstharz EM (trade name for a ketone resin prepared by the condensationof an aliphatic ketone with formaldehyde, marketed by RheinpreussenG.m.b.H., Homberg, Germany Esterified colophony Syntex 800 trademark fora cyclic rubber from the N.V. Chemische Industrie Syrnes, Hoek vanHolland, Netherlands Polyol X-450 (trademark for a copolymer of thefollowing formula:

marketed by lShell Chemical Corporation, New York, N.Y., U.S.A.)

Co[N-vinyl car-bazole/ethyl acrylate] prepared as described in UnitedKingdom patent specification 964,875

Polyvinylchloride e.g. Hostalit C 270 (trademark of Farbwerke HoechstA.G., Frankfurt and Main-Hochst, W. Germany) Piccolastic D-100 (tradename for a thermoplastic styrene polymer marketed by PennsylvaniaIndustrial Chemical Corporation, Clairton, Pa., U.S.A.).

The ratio of insulating binding agent and photoconductor depends on therequired quality of the photoconductive layer with respect to thephotoconductive properties, mechanical strength capacity and insulatingpower. Good results are obtained with a ratio of binding agent andphotoconductor of 1:3 to 1:9. When using layers with a relatively highcontent of binding agent, the image sharpness diminishes and when usinglayers with a much lower content of binding agent, the relaxation timeof the charge diminishes quickly.

To layers containing binding agents which are not sufficientlyhydrophobic by themselves, the desired waterrepellent properties can beconferred as known per se by special additives or by an after-treatment.Additives enhancing the hydrophobicity e.g. stearic acid as described inthe United Kingdom patent specification 883,783, or binding agentsbearing active hydroxyl groups which react with diisocyanates, asdescribed in the United Kingdom patent specification 896,610, can beincorporated in the photoconductive layer.

If polystyrene-butadiene latexes are used as binding agents, a thermalafter-treatment can be applied as described in the United Kingdom patentspecification 766,- 979.

The surface of the photoconductive layer can also be made morehydrophobic by adsorption of an appropriate substance on thephotoconductive layer.

It is also possible to make the surface of a photoconductive layer morehydrophobic by applying a coating layer comprising a hydrophobiclayer-forming material such as a hydrophobic polymer, varnish or wax.

If the photoconductive layer is more hydrophobic than is required, alayer (which may be very thin) of a less hydrophobic substance can beapplied thereon or the surface can be modified by applying asufficiently small quantity of a hydrophilic colloid, e.g., gelatin,polyvinylalcohol, a cellulose derivative or an alginic acid derivative.The thickness of such hydrophilic layer preferably varies between 0.2and 2 This layer can also be applied from a strongly diluted solution ofa wetting agent, but should not enable the surface charge to be carriedoff.

The photoconductive layer and/or a top layer for increasing thehydrophobic properties may includes other known additives such asplasticisers, dispersing agents, optical bleaching agents, substancescounteracting oxidation and aging, agents improving the gloss, mattingagents, sensitizing dyes and chlorine containing polymers which increasethe sensitivity as described in our United Kingdom patent specification964,878. The amount and the nature of these substances are chosen insuch a way that the chargeability of the layer is not markedlydiminished.

In the course of our experiments it has been found that particularlygood results can be obtained in the formation of a liquid record bymeans of an electrostatic charge record if the electrostatic chargecarrier is a photoconductive layer comprising photoconductive zinc oxideas a photoconductive component. The photoconductive zinc oxide/bindercomposition preferably consists for at least 50% of zinc oxide in ahydrophobic binder. Specially good results are obtainable with zincoxide having been prefer-ably treated with a suitable acid compound orsalt which increases the dark-resistivity of the zinc oxide. Theincrease of the dark-resistivity of the photoconductive zinc oxidepermits the formation of a photoconductive layer, which comprises thezinc oxide dispersed in a binder and in which the specific resistivityof the binder is not markedly higher or is even the same as or lowerthan that of the acid-treated zinc oxide.

By usual photoconductive zinc oxide is understood any commerciallyavailable type of zinc oxide which has been prepared according to theFrench process, in this process zinc oxide is prepared by oxidation ofzinc vapour.

Zinc oxide types which appear from our experiments to give particularlygood results are e.g.:

Blanc de Zinc, Neige Extra Pur, types A, B and C, marketed by VieilleMontague S.A., Liege, Belgium,

Zinkoxd (reinst) marketed by E. Merck A.G., Darmstadt,

Germany,

Florence Green Seal Lead-Free Zinc Oxide marketed by the New Jersey ZincCompany, New York, N.Y., U.S.A.

Zinc Oxide Analytical Reagent, marketed by Mallinckrodt Chemical Works,St. Louis, Mo., U.S.A.

The preferred compounds with acid properties for treating thephotoconductive zinc oxide are:

Aliphatic non-substituted monoand dibasic carboxylic acids or aliphaticmonoand dibasic acids which may contain one or more hydroxyl groups suchas lactic acid and tartaric acid, and Organic phosphorus compoundscorresponding to the general formulae:

wherein:

R represents a hydrogen atom, a hydroxyl group or a chlorine atom,

R represents a hydroxyl group, a chlorine atom, an alkyl group, asubstituted alkyl group, an aryl group, a substituted aryl group, analkoxy group, a substituted alkoxy group, an aryloxy group or asubstituted aryloxy group, and

R represents an alkyl group, a substituted alkyl group, an aryl group, asubstituted aryl group, an alkoxy group, a substituted alkoxy group, anaryloxy group or a substituted aryloxy group.

The following further elucidation of the invention includes adescription of further factors which can usefully be taken into accountin preparing the electrostatic charge carrier and in selecting thecomposition of this carrier and of the liquid.

As described on pages 394-395 of the aforesaid book of Schwartz, Perryand Berch, it is explained that there is a relation between theroughness of the surface of a solid material and the contact angle witha particular liquid. It appears therefrom that the size of the contactangle, with other words, the wetting power, may be influenced byadapting the roughness of the surface.

The degree of roughness in the structure of the surface of a chargecarrier in the form of a photoconductive layer comprising a dispersedphotoconductor may be influenced by suitably chosing the grain size andthe degree of dispersion of the photoconductive material or by thecontrolled flocculation of the binding agent in some solvents. Further,a granular structure of the surface can be obtained by adding compoundsto the photoconductive layer.

Further a certain rough surface structure can arise by applying thephotoconductive layer onto a screened support. Surface roughness canalso be obtained by applying the photoconductive layer in a regular orirregular screen form as e.g. by applying with a screened roller or bypressing a screen profile into the half-dry or plasticized thermoplasticmaterial of the photoconductive layer or a superimposed layer.

The surface roughness of a layer, i.e. the degree of the unevennesscs,may be determnied with a Perth-O-Meter (ref. Dr. Perthen,Hannover/Hommel-Werke, Mannheim). It is expressed by the sum of twoterms, namely W +Rt wherein W (Welligkeit is the depth measure for themacro-unevennesses of the surface and Rt is the depth measure of themicro-unevennesses of the surface. Macrounevenness here refers to thedeviations of the general surface contour from a plane andMicro-unevenness refers to the surface irregularities which are presentlocally in successive parts of the surface contour.

The total value W+Rt of the surface roughness is preferably comprisedbetween 2 and 15 The degree and kind of surface unevenness which ispermissible depends in part upon the way in which liquid is to be madeavailable for wetting the surface. The liquid must have proper access toall parts of the surface. When the liquid is applied from a roller ontothe charge carrier, attention should be paid to the distance between thefree liquid surface on the wetting device and the deepest point of anyrecesses or depressions or reticulations in the surface to be developed.Good results have been obtained with distances comprised between 1 andIOU/1., preferably between 3 and 30 If the distance is taken too small,e.g. by exerting too strong a pressure, the charge image is destroyed.If too big a distance is taken, the fine image details are lost. Theadjustment of the suitable distance between the liquid surface and theprofile of the material depends on the surface structure of the materialand the kind of the developing liquid.

If the developing liquid is brought near the surface to be developed atnormal atmospheric pressure, the surface roughness will lie preferablyin the average of the above given extremes. The optimum value of surfaceroughness for obtaining a favourable contact angle is shifted to higheror lower values, according whether the wetting with liquid isfacilitated or inhibited by one or more of the following factors:viscosity and surface tension of the liquid, surface roughness andsurface tension of the material to be developed, interfacial tension ofthe insulating material/liquid system, the quantity of the conveyedliquid, the pressure exerted upon the liquid, gravity, capillary forces,streaming potential (see Electrical Phenomena at Interfaces in ChemistryPhysics and Biology by J. A. I. Butler Co. Ltd. London (1951) pp. 77-82) and magnetic and electrical forces which arise between the materialto be wetted and the wetting liquid.

As aforesaid, the contact angle formed by the developing liquid with theuncharged insulating material, has to be such that the liquid on oropposite to the uncharged areas or on or opposite to the image areaswith insufficient field strength does not or does not sufficiently undergo electrostatic influence which causes spreading out or wetting of thesurface. On the other areas, on the other hand, the electrostatic fieldat the surface of the insulating material performs the work necessaryfor wetting.

The work required for Wetting can be increased or decreased by modifyingthe physical properties of the liquid and of the insulating material tobe wetted, e.g. by modifying surface tension, electric conductivity,polarizability, capacity or viscosity.

In order to obtain a visible image, the conductive aqueous medium maycontain preferably in solution or in dispersion dyestuffs which are fastto light. Agents for improving the fastness to light of the dyestuffsand mordanting agents may be added. Further, compounds may be added tothe liquid which improve dissolving, emulsifying and dispersing ofsubstances which influence the physical and chemical behaviour of theliquid. These substances comprise organic or mineral dyestuffs,substances which increase or decrease the surface tension and/ or theconductivity and/or the polarizability and/ or the capacity and/or theviscosity, binding agents e.g. colloids and latexes, macromolecularcompounds, substances improving the fastness to light of the dyestuffs,substances preventing the ink image from drying quickly. For the latterpurpose, a hygroscopic substance is preferably used if the liquid mediumis water.

In another embodiment an aqueous developing liquid is used whichcontains no dyestuff but a colourless or little coloured component,which is converted into a dyestulf only during or after the development,by oxidation with atmospheric oxygen, by exposing to light or heat, orby reaction with a reaction partner which is present in the support ofthe electrostatic image or (in case of transfer) in the ultimatesupport.

As we have already explained, processes according to the invention arepreferably performed using an aqueous medium consisting for at least 60%by weight of water. Very suitable developing liquids comprise water andone or more compounds of the following categories in the proportionsstated:

(1) 03-20% of a dyestuff soluble or dispersible in water.-Suitableorganic dyestuffs are e.g.:

Crystal Violet C.I. 42,555 Malachite Green C.I. 42,000 Methylene BlueC.I. 52,015 Victoria Blue 0.1. 42,595 and CI. 44,045 Carmine Red -C.I.75,470 Nigrosine C 140 powder C.I. 50,420 Chloramine Black EX (dark)C.I. 30,235 Rayon Black C (double cone.) (3.1. 35,255 Chris Cuprofles 3LB C.I. Direct Black 63 Suitable inorganic pigments are any structuralform of carbon e.g. graphite, carbon black, lamp black, bone black,charcoal, Ultramarine blue, cadmium sulfide, titanium dioxide, zincoxide, iron oxide, magnetic iron oxide, aluminum powder, and bronzepowder.

(2) Colourless or little coloured compounds conversable to a colouredcompound.-Such a colourless or little coloured compound can be convertedinto a coloured compound, during or after wetting the charge carrier,with a substance present in or on the material to be developed. Suitablereaction partners the reactivity of which is promoted by the presence ofa liquid phase are e.g. those described in United Kingdom patentspecification 898,354.

Suitable known colour reactions in aqueous medium are e.g. couplingreactions of diazonium compounds with known coupler compounds such as[i-naphthol.

For the colour formation in situ ferric salts are suitable which reactwith aromatic compounds containing hydroxyl groups e.g, pyrogallol anddodecyl gallate.

Suitable colourless reaction partners are the colourless triazolium andtetrazolium compounds such as those described in the British patentspecification 670,883. These compounds are converted into a colouredcompound by a reducing reaction partner.

The classical colour coupling reactions between oxidiz' able aromaticamino developers and colour couplers are also applicable. Classicalcolour coupling reactions are described e.g. in The theory of thePhotographic Process by C. E. Kenneth Mees, revised ed., 1954, TheMacmillan Company, New York, pages 584-589.

Many other colour reactions and bleaching-out reactions of colours,which also may be used for forming an outlined image pattern, are knownto those skilled in the reproduction art. Further, an extensive list ofcolour reactions is set out by Feigl in Spot Tests 1954, ElsevierPublish. Corp, Amsterdam.

It is also possible, provided a suitable solvent or dispersing agent forpolyvinyl chloride is used, to cause a colour reaction with zinc oxide,after the deposition of a polyvinyl chloride residue on the chargedareas of a layer containing zinc oxide by Warming the polyvinylchloride.

It is evident that an applied colour reaction may be activated oraccelerated by heat and light, and that in the layer with theelectrostatic pattern as well as in the developing liquid catalyst knownper se can be incorporated which promote the colour reaction, ifnecessary.

The formation of a visible image by reacting of one or more reactionpartners in the material surface to be developed, and which isattainable for the reaction with one or more reaction partners in aliquid phase, has the advantage that the formed image is well anchoredin the surface of the material so as to be very resistant to mechanicalerasure.

(3) 0.220% of a substance influencing the surface tension-The surfacetension may be increased by adding water-soluble substances e.g.potassium carbonate, aluminium sulfate, iron sulfate, cadmium chlorideand magnesium sulfate. A list of other such substances as given e.g. inTaschenbuch fiir Chemiker und Physiker by J. DAns and E. Lax, edition1949, Springer Verlag, p. 1008.

The surface tension may be decreased by adding Watermiscible organicsubstances e.g. methanol, ethanol, acetone, methyl ethyl ketone, aceticacid, hydroquinone, lauryl sulfonate, dodecyl sulfonates, saponine andpolyglycol derivatives.

Other useful surface-active substances are given in Textilhilfsmittelund Waschrohstoffe by K. Lindner, Wiss. verlagsgesellsch. m.b.H.,Stuttgart 1954.

(4) Up to 10% of a substance which delays drying up of the ink.e.g.glycerol, glycol and sorbitol.

(5) Up to 10% of a water-soluble or water-dispersible blinding agente.g.gum arabic, carboxymethyl cellulose, casein, polyvinylpyrrolidine,polyacrylates, polystyrene, polyvinylacetate, waxes, silicates andcolloidal silicic acid. These substances decrease ability of the dry inkof being washed away by water.

(6) Up to 35% of an organic polar liquid with high dielectric constantand miscible with water.- e.g. formamide. According to the choice of theused dyestuffs (if any) in the insulating layer and in the ink,different colour contrasts may be obtained. If the colour of the ink isdarker than that of the insulating layer, a positive image is obtained.If the colour of the ink is lighter than that of the insulating layer,counter image (positive print of a negative original) is obtained. Inthe latter case, a photoconductor which is dark-coloured in itself isused e.g. selenium, cadmium sulfide, cadmium selenide and antimonysulfide, or a white photoconductor e.g. zinc oxide, to which sensitizingor other dyestuffs are added. Development can than be carried out with adispersion of a white or clear-coloured pigment e.g. titanium oxide,zinc oxide, zinc sulfide, barium sulfate, antimony oxide, china clay andcalcium carbonate. Counter images can also be formed by using as chargecarrier an insulating layer comprising at least at its surface a darkdyestuff and by using an aqueous solution of a bleaching agent for thisdyestuff, for the development.

With reference to embodiments of the invention in which theelectrostatic charge carrier is a photoconductive layer which has arelaxation time of preferably at least 0.1 second, and which is unitedwith a conductive support. Suitable conductive supports are e.g. platesor foils of metal such as aluminum, copper, bronze, lead and zinc, orglass-plates provided with a thin layer of tin oxide having a specificresistivity of 10 to 10 ohm.cm., foils or fabrics of plastic substances,provided with a thin conductive layer such as described in UnitedKingdom patent specification 964,873 and finally paper. Suitable kindsof paper are those which show a resistivity lower than 10 ohm-cm. at arelative humidity of 50% e.g. the kinds of paper described in UnitedKingdom Patent 995,491. Other suitable kinds of paper are thosecontaining at least 2% of conductive filing materials e.g. carbon. Othersuitable papers are those of which the surface, which is turned to thephotoconductive layer, is provided with a conductive layer e.g. a thinlead or aluminium foil, or a dispersion of a. metal powder or of carbonpowder in a binding agent. Finally the fabrics consistingpreponderatingly of carbon e.g. those marketed by Union CarbideCorporation, New York, N.Y., U.S.A. under the name Graphite Cloth GradeWCA, WCB and WCC, may successfully be used as conductive supports.

Fabrics or kinds of paper which show insufiicient conductivity at theprevailing air humidity, may also be used successfully, provided thatthe rear side of the support is wetted with Water before or during thedevelopment.

Printing plates can also be manufactured according to the invention. Forexample an electrostatic latent image can be developed with an aqueousdispersion of a hydrophobic binding agent and the moist image thentransferred to a gelatin foil and dried. After wetting the gelatin andrubbing with printing ink, only the areas covered with binding agentsaccept ink. Suitable dispersions of hydrophobic binding agents arelatexes e.g., polystyrene latex, latex of cobutadiene/acrylonitrile andLytron S-lO (trademark for a thermoplastic copolymer of styrene marketedby Monsanto Chemical Company, Springfield, Mass., United States ofAmerica).

Reference is now made to the accompanying drawings by way of explanationand illustration of the invention. In these drawings:

FIGS. la and lb and FIGS. 2a, 2b and 2c are explanatory of thephenomenon of surface wetting and the way in which the inventionutilises this phenomenon;

FIG. 3 is a diagrammatic representation of a device for use in carryingout the invention. In this figure however the means for applying thesuperimposed electric field is not represented;

FIG. 4 represents stages in the formation of a reversed record (counterimage).

Referring firstly to FIGS. 10, 1b, 2a, 2b and 2c, the first two of thesefigures show respectively a drop deposited on a surface which is wettedby the liquid and a drop deposited on a surface which is substantiallyunwetted by the liquid. The so-called contact angle is the angle betweenthe surface on which the drop is deposited and a tangent to the curve ofthe liquid drop drawn through a point where the drop curve meets thesurface. In the case of the first drop the contact angle is much lessthan 90 while in the case of the second drop the contact angle issubstantially greater than 90.

FIG. 2a shows graphically how the contact angle of an aqueous developingliquid on the hydrophobic surface of an electrophotographic layer isinfluenced by an electric charge. After corona-charging the layer has,e.g., at its surface, a field strength of 100 to 600 volt/cm. and afterexposure a field strength between and 100 volt/cm.

FIGS. 2b and 2c diagrammatically represent the behaviour of a drop ofaqueous medium 3 on the hydrophobic surface of an insulating layer 2borne by a conductive support 1, before and after charging. FIG. 2bcorresponds with point b of the curve of FIG. 2a and FIG. 20 correspondswith point c of such curve. At the charge level represented by point cthe liquid drop which is normally repelled by the surface and leavesthis substantially unwetted, becomes practically completely spread out.

Referring now to FIG. 3, the apparatus here represented comprises aroller 10 which rotates in a container 9 filled with aqueous ink 6. Aphotoconductive layer bearing an electrostatic charge pattern isadvanced between a smooth surface roller 10 and a pressure roller 11.The output of liquid can be regulated by means of a roller or by meansof a doctor knife 17 as shown.

The liquid development is influenced by applying during the passage ofthe foil 5 between the rollers, an electric field perpendicularly to thefoil. The means which applies the electric field is not shown but infact a source of is connected to the rollers 10 and 11. A tension of10100 volts, the field direction of which is opposite to that of thecharged photoconductive layers, increases the contrast. A tension of-150 volts with a field of the same direction as that of the chargedphotoconductive layer reverses the image, i.e., liquid is deposited ontothe exposed areas.

By applying an alternating field perpendicular to the foil, e.g., byconnecting rollers 10 and 11 to the terminals of an alternating voltagesource, the image quality can be improved. Both the magnitude of thealternating voltage and the frequency affect the results. If at arelatively low frequency, e.g., 50 cycles, a little ink is found todeposit on the unexposed areas, this soiling of the image background canbe avoided or reduced by increasing the frequency.

The rate of conveying the photoconductive material between the rollerscan vary between wide limits. Good results have been obtained withspeeds of the photoconductive material between 1 cm./ sec. and 1 m./sec.

Reference is now made to FIG. 4. According to the process hererepresented, a latent electrostatic image on a charge carrier (FIG. 4a)is developed to the invention with an aqueous conductive solution,emulsion or dispersion (FIG. 4b) which after drying leaves a hydrophobicresidue (FIG. 4c) which is electrically conductive. For this purpose, anaqueous dispersion of modified paraflin as marketed by Mobil Oil A.G.,of Hamburg, W. Germany under the trademark Mobilcer A is suitable. Afterdrying, charging is repeated once again (FIG. 4d). Only the uncoveredareas of the carrier retain electrostatic charges. Development now takesplace with an aqueous dyestutf solution (FIG. 40) which wets only theareas made hydrophilic by the previous charging.

Ink images obtained according to the invention may be transferred ontoanother support in a simple way, by bringing the still wet ink images incontact with an inkaccepting surface e.g. a porous surface such as paperor a transparent foil covered with gelatin. This process permits e.g. toreproduce documents according to the reflectographic exposure method,whereby a positive mirror image is obtained which is transferred ontoanother support so as to form a positive legible image.

Finally it has to be mentioned that the still wet ink image may betransferred onto the back-side of the support by rolling up thedeveloped image still wet around a cylindrical rod of small diameter. Inthis way an ink print is transferred onto the back-side of the wettedmaterial. This method is especially interesting to obtain a mirror imageof a text. In this method good results have been obtained, especiallywhen using those electrophotographic materials according to the presentinvention which comprise a thin paper support with a certain sur faceroughness.

The transfer of a colour image to a transparent support in the presenceof a mordanting agent for the dyestuff is specially suitable forproducing images in more than one colour. Suitable dyestuffs andmordanting agents for this process are described e.g. in the -UnitedStates patent specification 1,121,187. The procedure is the following:Thre colour separation records are formed by exposing three sheets of anelectrophotographic material to a coloured original, one through a cyan,one through a magenta and the other through a yellow filter.

Photoconductive substances the spectral sensitivity of which is suitedfor the manufacture of electrophotographic materials used for thereproduction of colour images are e.g. selenium to which arsenic ortellurium are added or zinc oxide to which sensitizing agents are addedas described in the United Kingdom patent specifications 885,718;895,723; 954,017; 954,018; 885,715; 885,716 and 885,717.

13 The three selection images are developed according to the inventionwith an ink which has the same absorption spectrum as the used filter.These ink images are then successively transferred in exact register tothe same foil, so as to obtain a true colour reproduction of theoriginal.

The invention may further be applied successfully in different printingtechniques e.g. in hectography, offset printing and for obtaining etchreliefs. In a hectographic process, a latent electrophotographic imageis developed with a concentrated dispersion of a dyest-uif e.g. anaqueous dispersion of crystal violet or malachite green. After drying,the developed image is employed in an alcohol duplicating machine tomake the desired number of copies.

For manufacturing offset printing plates, a latent electrophotographicimage is developed with a dispersion which after drying results in ahydrophobic residue. Then the material is braced on an offset machineand the image background is made hydrophilic e.g. with ferricyanide andorganic acids.

For manufacturing etch reliefs, an electrophotographic material is usedwhich in its photoconductive layer contains a polymer with free hydroxylgroups (e.g. the copolymer available under the trademark Polyol X-450hereinbefore referred to) as a binding agent. The electrostatic latentimage is developed with an aqueous dispersion of a substrate (hardeningagent) which is able to react with these hydroxyl groups. The hardeningagent either can be incorporated into the ink or dispersed in thebinding agent and a catalyst for promoting the hardening reaction may bedissolved in the ink.,. Suitable hardening agents for these polymers aree.g. urea derivatives such as dimethylol urea described in the BritishPatent specification 580,275, or monocyclic ureas, described in theUnited Kingdom patent specification 575,260. After drying, theelectrophotographic material is heated for a short while at 100-150 C.Hereby a reaction takes place on the image areas between the bindingagent and the hardener, whereby on these areas the solubility of thephotoconductive layer is diminished markedly. Now the non hardened imageparts can be removed with a suitable solvent whereupon the metal supportcan be etched through the etch resist in a known way.

The following examples illustrate the present invention.

EXAMPLE 1 To 500 cos. of a 4% solution of Flexbond D-13 (trademark) inethanol, 225 g. of photoconductive zinc oxide (neige extra pure, type A,marketed by Vieille Montagne S.A., Liege Belgium) are added. The mixtureis ground in a ball-mill for 48 h., whereupon the following compositionis added whilst thoroughly stirring:

Ccs. 2% solution of Flexbond D13 (trademark) in ethanol 500 solution ofmonobutyl phosphate in ethanol 10 10% solution of succinic acid indimethyl form amide 10 1% solution of fiuoresceine (C.I. 45,350) inethanol 10 This photoconductive dispersion is applied by knifecoatingonto a paper support provided with an aluminium foil at a rate of 10 sq.m. per liter, and dried.

The obtained photoconductive layer is charged up to a tension of --300v. by a tension of --7000 v. at the corona wires, and exposed for 0.7sec. to a diapositive with a 75 watt bulb placed at a distance of 10 cm.Then the latent image is developed with a developing device as shown inFIG. 3 which is explained below, by means of a developing inkcontaining:

A 15% colloidal graphite dispersion having a specific conductivity of8.2x 10- mho/cm g 5 Ludox (trademark for a 30% aqueous dispersion ofcolloidal silicic acid marketed by E. I. du Pont de Nemours & Co.Wilmington, Del., U.S.A.) g 15 Water ccs 10 The apparatus shown inFIGURE 3 comprises two rollers and an ink container. Roller 11 is analuminum cylinder with a diameter of 15 mm. and a length of 25 cm.,which accomplishes the function of guiding roller for the material to bedeveloped. Roller 10 is a smooth chromiumnickel steel 18/8 cylinder witha diameter which is twice as large as that of roller 11. The rollers 10and 11 are connected to a v. direct current source so that duringdevelopment a direct current field is superimposed normally to the foil5, the direction of the field being opposite to that due to theelectrostatic latent image.

The photoconductive layer of the exposed material is contacted with theink roller and held at a distance of 0.5 mm. so as to form an inkmeniscus of 3 mm. The photoconductive material is moved at a speed whichis equal to that of the ink roller. The roller 10 rotates at a speed of2 m./ min. and is partly immersed in the developing ink.

The ink settles onto the nondischarged image areas and forms afterdrying a positive non-washable image.

EXAMPLE 2 Onto a photographic paper support the following conductiveprecoating layer is applied:

A 30% aqueous dispersion of colloidal carbon, the

carbon particles of which have an average diameter of 0.1 and whichpossesses a specific conductivity of 10- mho/cm g 50 2% aqueous solutionof carboxymethyl cellulose ccs 500 Water ccs 100 Ethanol ccs 400 Aphotoconductive layer as described in Example 1 is applied thereon.

After charging the layer as in Example 1, an image is episcopicallyprojected thereon for 30 sec. For this purpose two Sylvania Green(trademark) exposure lamps of 14 watt and a Schneider Kreuznach Xenar(trademark) 4.4 lens with a focal distance of 21 cm. are used, placed ata distance of 84 cm. and set at a diaphragma aperture of 5.6.

The positive latent electrostatic image is developed in the same way asin Example 1 but with a developing liquid of the following composition:

Ccs. 15% aqueous dispersion of colloidal carbon having a specificconductivity of 82x10" mho/crn. 40

Water 100 For increasing the density of the developed image, therotation speed of the ink roller is increased to 20 m./min. and thetravelling speed of the paper to 3 m./min. The distance between thephotoconductive layer and the roller is 1 .mm. A vigorous positive imageis obtained.

EXAMPLE 3 The following composition is ground for 24 hours in aball-mill:

Zinc oxide (from Example 1) (neige extra pure, type A, marketed byVielle Montagne S.A., Liege, Belgium) k-g 4.5 4% solution of FlexbondD-13 (trademark) in ethyl alcohol litres 9 15 After grinding the mixtureis diluted with the following mixture:

4% solution of Flexbond D-l3 (trademark) in ethyl alcohol litres 21 10%solution of monobutyl phosphate in ethyl alcohol -Q ccs 300 10% solutionof succinic acid in dimethyl formamide ccs 300 1% solution offiuorescein in ethyl alcohol ccs 300 This dispersion is coated onto abaryta-coated paper of 90 g./sq. m. according to the knife-coatingsystem and in such a way that with 1 litre 10 sq. m. are covered.

After drying, a uniform charge to a tension of 400 v. is applied onthese layers by means of a corona discharging of 6000 v. Then thecharged layer is image-wise exposed for 0.7 second, through a negativetransparency with a lamp of 75 w. placed at a distance of 10 cm. Theformed latent image is developed in the following dyestuff solution:

G. Methylene blue 2 Water 100 having a specific conductivity of 1.2 10-mho/cm., by means of the apparatus as described with reference to FIG. 3while an alternating tension of 100 v. and 50 cycles is applied betweenthe rollers and 11 during development. The contact time of liquid andphotoconductive layer was 0.1 second. A positive legible image isobtained.

At 50 cycles a vigorous image is obtained but a little ink is alsodeposited upon the non exposed areas. This undesired ink-deposit can beremoved by increasing the frequency of the applied tension duringdevelopment, so that already at 1000 cycles a positive image with a purebackground is obtained.

Only in the areas wherein wetting really occurs, so wherein the airadsorbed to the charge carrier is replaced by the polarizable conductiveliquid discharge of the surface charge pattern can take place.

The charge built up by corona discharge in fact does not consist of freeelectrons but rather of adsorbed negative oxygen ions created by theionization of the air. The desorption and discharge of said ions takestime in the order of tenths of seconds so that a suflicient timeinterval for development e.g. with rollers exist to build up anelectrical double layer between the polarizable liquid and thecorona-charged layer.

Of course in the case a P.I.P. charge pattern has to be developedaccording to the present liquid contact development method no problem inrespect of a critical developing time arises.

We claim:

1. A process of producing liquid images in which a normally insulatingelectrostatic charge carrier having a hydrophobic surface and bearing anelectrostatic charge pattern detectable at such surface is developed bywetting with a developing liquid according to such charge patternthereby to record such pattern by means of surface Wetting, whichincludes the step of substantially uniformly contacting the hydrophobicsurface of said carrier, progressively or simultaneously, with anaqueous conductive developing liquid in the presence of a superimposeddirect current electric field extending generally at right angles tosaid surface, said surface being normally unwettable by said aqueousdeveloping liquid but selectively wettable thereby under the influenceof attractive electrostatic field forces, the period of said contactbeing less than the discharge time of the charge pattern on contact ofthe charge carrier with the developing liquid.

2. A process of producing liquid images according to claim 1, whereinthe total electrostatic field forces created by said charge pattern andsaid external field is at least about 100 volts.

3. A process according to claim 1 wherein the said superimposed directcurrent field has a polarity opposite to that of the field created bysaid electrostatic charge pattern and the liquid wets the surface ofsaid charge carrier in the area bearing said electrostatic chargepattern.

4. A process according to claim 1 wherein the electrostatic chargecarrier is a normally insulating photoconductive layer that afteruniform electrostatic charging and imagewise exposure to electromagneticradiation to which said layer is sensitive, contains a charge pattern,said layer having a hydrophobic surface, and wherein said superimposeddirect current field has the same polarity as the field created by saidelectrostatic charge pattern and causes the liquid to wet said surfacein the exposed and uncharged areas thereof and to be repelled in theunexposed, charged areas thereof.

5. The process of claim 1 wherein said hydrophobic charge carriersurface has a total surface roughness of about 2l5,u..

6. A process of developing a normally insulating electrostatic chargecarrier having a hydrophobic surface and which exhibits rectifyingproperties when placed in an alternating current field, said carrierbearing an electrostatic charge pattern, which comprises the step ofcontacting said surface substantially uniformly and within asuperimposed alternating current electric field directed generally atright angles to said surface, with an aqueous conductive polarizableliquid by which said surface is normally unwettable but by which suchsurface is selectively wettable under the influence of attractiveelectrostatic field forces, whereby said carrier is wetted with liquidaccording to such charge pattern thereby to record such pattern by meansof surface wetting.

7. A process according to claim 6 wherein said alternating current fieldhas a frequency substantially above 50 cycles per second, and the totalelectrostatic field forces created by said charge pattern and saidsuperimposed field is at least about volts.

8. A process according to claim 7 wherein the alternating current fieldhas a frequency of more than 500 cycles.

9. A process according to claim 6 wherein said charge carrier includes anormally insulating photoconductive layer comprising photoconductivezinc oxide grains dispersed in a hydrophobic binder, forming ahydrophobic surface on said layer, and said electrostatic charge patternis created by means of corona discharge.

10. A process according to claim 9 wherein the photoconductive layercomprises the reaction product of photoconductive zinc oxide grains anda monoor di-basic aliphatic carboxylic acid or organic phosphorouscompound.

11. A process according to claim 6 wherein said developing liquid isconductive and said electric field is maintained between said conductiveliquid and a relatively conductive backing for said charge carrier, saidconductive liquid and said backing being connected to the opposite polesof an external alternating voltage source, said backing being insulatedagainst electrical contact with said conductive liquid by means of thenormally insulating charge carrier intervening therebetween.

12. The process of claim 6 wherein said hydrophobic charge carriersurface has a total surface roughness of about 2-15 References CitedUNITED STATES PATENTS 3,076,722 2/1963 Greig 11737 3,080,251 3/1963Claus 961 3,084,043 4/1963 Gundlach l17-37 3,096,198 7/1963 Schaffert1l737 X 3,102,045 8/1963 Metcalfe et a1. l17-37 3,106,157 10/1963Reithel 117-37 X (Other references on following page) UNITED STATESPATENTS Cassiers et a1. 961 Middleton et a1. 96-1 Blake et a1. 961Castle 96-1 Cassiers et a1. 96-18 Crumley et a1 96-1 Kurz 96-1 Metcalfeet al. 117-37 Olson 117-37 Richer 117-37 Mayer 117-175 X Schaflfert118-637 Greig 961 Reithel 117-37 X Greig 117-37 Gundlach 117-37 Murray96-1 X Walkup 961 X Staicopoulos 260-41 Mayer et a1. 117-17.5 X

Mayer 117-175 X FOREIGN PATENTS Great Britain. Great Britain.

OTHER REFERENCES Bikerman: Surface Chemistry, pp. 422-423, 1948.Schwartz-Perry: Surface Active Agents, pp. 473-76,

WILLIAM D. MARTIN, Primary Examiner E. J. CABIC, Assistant Examiner US.Cl. X.R.

